A vehicle is a transportation means that produces power from an engine equipped therein and transfers the power to the wheels to carry passengers and freight on a road. A vehicle is generally divided into a body forming the external appearance of the vehicle, and a chassis that organically connects various devices to each other. The chassis includes an engine of the vehicle, which produces driving force for travel, and main apparatuses such as a power transmission, a steering system, a suspension system, and a brake system.
The engine produces the driving force to move a vehicle. Most vehicles are provided with a four-stroke internal combustion engine. The four-stroke internal combustion engine, which completes one cycle consisting of four strokes of intake, compression, combustion, and exhaust, is the most common example of a reciprocating engine. Internal combustion engines, which mainly use a volatile fuel, obtain kinetic energy using thermal energy which is generated when the fuel mixed with the oxygen in the air to allow complete combustion is compressed and combusted.
Such internal combustion engines using a volatile fuel cause environmental pollution due to exhaust gas and the depletion of petroleum resources. As an alternative, electric vehicles powered by electricity have emerged.
Electric vehicles (EVs) are mainly powered by driving an AC or DC motor using the power from a battery. The EVs are generally classified into battery-only EVs and hybrid EVs. Battery-only EVs drive a motor using power from a battery, which is recharged once remaining battery charge approaches a lower limit. Hybrid EVs can recharge a battery by operating an engine and generating electricity and move by driving the electric motor using this electricity.
Hybrid EVs can further be classified into a serial type and a parallel type. Serial type hybrid EVs are vehicles that are always driven by a motor by converting mechanical energy output from an engine into electric energy through a generator and supplying this electric energy to a battery or the motor. The serial type hybrid EVs are configured by adding an engine and a generator to conventional EVs to increase range. Parallel type hybrid EVs can be driven by power from two power sources, i.e., can be driven either by power from a battery or by an engine (a gasoline engine or a diesel engine). Parallel type hybrid EVs can be driven by both an engine and a motor depending upon travel conditions.
Therefore, EVs powered by batteries alone and hybrid electric vehicles (HEVs), which use a battery and an existing combustion engine battery together, have been developed. Some EVs and HEVs are already commercially available. As a secondary battery serving as a power source of the EVs and HEVs, a nickel metal hydride (Ni-MH) cell is mainly used, but in recent years, use of lithium-ion cells has also been implemented.
In order to be used as a power source of EVs and HEVs, high power and high capacity is required. Accordingly, a medium scale or large-scale battery pack formed by connecting a large number of small secondary batteries (unit cells) in series and/or in parallel is used.
As a unit cell constituting medium scale or large-scale battery pack, a prismatic battery or a pouch cell, which can be stacked at high density to reduce dead space, is used. To facilitate electrical connection and mechanically fastening of such unit cells, a cell cartridge on which one or more unit cells can be mounted is used. That is, the battery pack is configured by stacking a plurality of battery cell modules having unit cells mounted thereon.
In recent years, research into battery management systems (BMSs) to determine the time to replace battery cell modules of an EV through recharging voltages of the battery cell modules is underway.